An optical coupler or “opto-coupler” (also referred to as an optical isolator or opto-isolator) is a type of microelectronic device in which a digital electrical input signal is converted to an optical signal, which is then detected to convert it back to a digital electrical output signal. An opto-coupler typically includes a light source such as light-emitting diode for converting the input signal to an optical signal, a light detector such as a photodiode for converting the optical signal to an electrical signal, and electronic circuitry associated with the foregoing elements, all formed on one or more semiconductor chips or dies contained within a device package. The device package has pins, i.e., electrical contacts, for the input and output signals.
In an opto-coupler designed for use in digital circuitry, it is desirable for the output signal to be as sensitive as possible to a change or bit transition in the input signal. Accordingly, it is desirable for the output signal to respond to the minimum or threshold signal level that results in the light source turning on or off. The output signal should thus transition between a digital “0” and a digital “1” as soon as the light source input signal reaches the minimum or threshold current necessary to turn it on or off. The sensitivity of an opto-coupler is difficult to control with great precision due to uncontrollable variations in semiconductor fabrication process parameters, materials, and the device assembly process (e.g., optical alignment between the light source and detector).
Trimming has been employed to adjust sensitivity or other device parameters of opto-couplers and other analog or partly analog devices so that a device parameter falls within a predetermined or manufacturer-specified tolerance. Trimming refers to a process of adjusting one or more analog electronic elements that affect the parameter sought to be adjusted. For example, a resistance can be adjusted to improve sensitivity.
Trimming can be performed before or after the device is assembled. For example, during the integrated circuit chip manufacturing process, after the integrated circuits have been fabricated on a wafer but before dicing the wafer into individual chips or dies (a stage sometimes referred to as wafer-level), electrical probes can be used to apply input signals to the circuitry on the wafer, read output signals, and trim the circuitry accordingly. By momentarily applying a high voltage to a fuse, i.e., a short metal link, formed on the wafer between a resistive element and circuitry to be trimmed, the fuse can be removed or transformed from an electrically closed state to an electrically open state, thereby altering the resistance experienced by the circuitry to be trimmed. Removing a fuse can provide fine changes in resistance, on the order of one ohm or less.
Analog integrated circuitry can also be trimmed after the device has been manufactured, using digital trimming logic included in the circuitry itself. The trimming logic can include non-volatile memory in which trimming values are stored. When the device is used in normal operation, the trimming values are read out of the memory and applied to circuit elements that affect parameters of the analog circuitry.